Vaccinia virus is used to vaccinate persons to prevent disease due to smallpox. Compared with other live virus vaccines, the smallpox vaccine is associated with frequent side effects including fever, a sore or swollen arm, headache, and fatigue. Other more serious side effects have also been reported including development of multiple lesions outside the vaccination site (generalized vaccinia), inflammation of the brain (postvaccinia encephalitis), severe infection of the skin in persons with a history of eczema (eczema vaccinatum), and disseminated vaccinia virus infection in immunocompromised persons (progressive vaccinia). Previously, we tested blood from smallpox vaccine recipients who received the Dryvax formulation of vaccine and found that 4 of 202 blood samples (from 3 of 27 vaccine recipients) were positive for the smallpox vaccine virus in the blood by PCR, but none were positive by culture. The lack of finding infectious smallpox vaccine virus in the blood indicates that the current guidelines for deferral of blood donation from smallpox vaccine recipients are appropriate. Since the study was begun, the Dryvax formulation of smallpox vaccine has been replaced by the ACAMBIS 2000 formulation. We are now testing persons who received the ACAMBIS 2000 formulation of vaccinia and will be determining if blood samples and throat swabs might contain virus. Last year, in collaboration with investigators at the Centers for Disease Control and Prevention and at Emory University, we reported a patient who developed progressive vaccinia after smallpox vaccination and cytotoxic chemotherapy for treatment of acute leukemia. The patient was treated with vaccinia immune globulin and two investigational drugs, ST-246 and CMX001 to slow the progression of the vaccinia virus infection and allow time for his cellular immunity to return after chemotherapy. The level of virus on swabs from the skin gradually declined as his T cell function approved, and the level of vaccinia virus-specific T cells in the blood gradually declined as his infection came under control. This case demonstrated the need for careful laboratory monitoring of patients with progressive vaccinia and the need for development of new anti-poxvirus drugs to manage these diseases.